RICHMOND, Va. - NASCAR will mandate a change in the design of the Car of Tomorrow in an attempt to alleviate the heat that caused foam to melt in several cars last weekend at Martinsville.

About 50 Nextel Cup Series teams were at Richmond International Raceway on Tuesday to begin two days of testing the Car of Tomorrow on a larger track.

Nextel Cup director John Darby said in an interview that before the next CoT race at Phoenix on April 21, NASCAR will require teams remove a 23 inch-by- 8-inch block of foam above where the exhaust pipes extend under the right side of the cars, and surround the area with a heat shield.

The idea is to create cooling air flow in the hottest area on the car and cut down on the melting of the foam that gave Matt Kenseth trouble in the CoT's first race at Bristol and helped cut short Kevin Harvick's race at Martinsville last weekend.

Harvick was not the only driver whose car had problems with smoke from the melting foam at Martinsville, but it was the only one of the three CoT Chevrolets fielded by Richard Childress Racing to have the issue, Darby said. Teams from Hendrick Motorsports and Kenseth's team, Roush Fenway Racing, also reported no problems this week, he said.

Some teams that haven't had issues with heat in the car or exhaust fumes have asked if they would be forced to make the changes, Darby said, and were told the changes would be mandated for every team.

"You may not have had a problem last week in Martinsville, but there may come a day when you slap the side of the wall and break a hole in the tailpipe or knock the tailpipe loose," he said. "What we tried to do is come up with a satisfactory fix so we could put all the cars back to a consistent format" that protects the drivers.

The inconsistency of teams that have had heat-related problems is explained by the different ways they set up their cars, Darby said. "It goes all the way to how an engine tuner may set the timing and adjust his air-fuel mixture," he said.

Harvick's car, for example, had a rich mix evident each time he entered a corner because it emitted a flash of flame from the tailpipe, likely from fuel. The heat in cars tends to increase and spread when they stop, Darby said, explaining why Harvick's car's most serious problems came after a 32-minute red-flag delay for rain.

Kenseth and Harvick both expressed concern that the smoke generated by the melting foam could be dangerous, and Darby said NASCAR has been assured it is not toxic.

The foam, however, melts at between 400 and 500 degrees, he said, while floor board and rollbar paint melts at around 250 degrees, and those fumes can be toxic.

After seven years in development, Darby said he remains pleased the issues with the CoT have been relatively minor and easily remedied since its rollout. The first week, most teams used exhaust pipes with metal that was too thin, causing the pipes to crack.

The cracks allowed carbon dioxide to build up in some cockpits, and several drivers finished the racy with headaches or lightheadedness from breathing unhealthy air.

That was fixed before Martinsville as teams used thicker exhaust pipe metal.

"If you look at the old car that's been around since 1981 and the fact that we're still fixing problems with it, to roll a brand new race car out and for the most part be trouble free, I'm pretty satisfied," Darby said.

On the track, drivers are still trying to figure out the new machine, which has been hyped as being safer while giving lower-budget teams a chance to better compete.

"We're like everyone else," Ricky Rudd said. "We're sort of searching for this Car of Tomorrow. We're not where we want to be."

The CoT has been tested only one other place this season - at Bristol before the race there - but the uniqueness of the half-mile layout made that test useful only for that track, Greg Biffle said. This test will have more wide-ranging benefit. Often described as racing like a small superspeedway, Richmond will allow drivers to reach speeds in the 120 mph and higher range, and that will allow them to glean information about how the CoT reacts at higher speeds like they will likely reach at 1-mile Phoenix International Raceway on April 21, the next race the CoT will be used. With many driver complaints about how difficult the CoT is to turn in the corners, the speeds reached at .75-mile Richmond will allow them to hopefully make gains. During the test, Juan Pablo Montoya and Scott Riggs both crashed. Montoya drove away after backing into the wall; Riggs went to the care center and was released. "This is going to be a place where we're going to have to work on it and working toward getting that front grip to get the car to go around the corner," Biffle said. For Toyota, which is making its debut in the Nextel Cup Series this year, the testing is vital, said Andy Graves, senior manager for Toyota Racing Development. "Every test that we attend and every day that we spend with these cars is a step forward," he said of the seven Toyota teams sitting outside the top 35 in points and having to qualifying their way into races. "Our teams are doing a good job of learning and making small gains each and every day to move forward. We'll just keep attacking it and working on it," Graves said. The test session was scheduled to continue today.